This application claims the priority of German Patent Application No. 197 54 013.9, filed Dec. 5, 1997, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a system and a process for the water vapor reforming of a hydrocarbon. Systems of this type are known, for example, methanol reforming systems for fuel-cell-operated motor vehicles that provide the hydrogen required for the fuel cells. This mobile application, in particular, requires a sufficiently rapid reaction to the typical fast load changes in vehicle operation. This application also requires the ability to supply hydrogen for the fuel cells as rapidly as possible after the respective system start and to achieve this by means of a reforming system that has a relatively compact construction.
The water vapor reforming reaction for reforming a hydrocarbon, such as methanol, takes place endothermally and at a reaction temperature that is raised in comparison to the room temperature. During a cold start of the reforming system, sufficient hydrogen can therefore not immediately be provided by means of the water vapor reforming reaction because the components of the system must first be brought to a corresponding operating temperature. Specifically with respect to the use in fuel-cell-operated motor vehicles, there is the desire to reach the warmed-up normal operation of the reforming system as fast as possible so that the fuel cells can be fed as early as possible with hydrogen generated during the driving operation. Various methods have been suggested for an accelerated cold start of reforming systems.
In U.S. Pat. Nos. 4,820,594 and 5,110,559, a conventional method consists of assigning a burner to the reformer within the reformer housing, in which the reforming reaction is to take place. The burner burns a combustible hydrocarbon/air mixture at an open flame in order to heat up the reformer. U.S. Pat. No. 5,110,559 also suggests that the generated hot combustion exhaust gases be passed into a CO shift converter which follows in order to also heat the CO shift converter.
In French Patent documents FR 1 417 757 and FR 1 417 758, during a cold start of a system for the water vapor reforming of methanol, a mixture of methanol and an oxidant is introduced into the reformer in order to initiate a corresponding combustion reaction and thus heat up the reformer. Then the feeding of the oxidant is terminated, the methanol/water vapor mixture to be reformed is fed, and the reforming reaction is started.
Japanese Laid-Open Application JP 07126001 A describes a compactly constructed reforming system in which an evaporator, a reformer and a CO-oxidant are provided which are part of a plate stack construction and are arranged serially behind one another in a transverse direction perpendicular to the longitudinal stack direction. The three system components can be heated in parallel by means of an assigned burner with pertaining heating layers of a laminar construction.
U.S. Pat. No. 5,516,344 discloses a compactly constructed reforming system in which a reformer and a CO layer converter are arranged underneath one another and divided by a separating plate from one another in a common housing. On the housing, a burner is provided by means of which a fed combustible mixture is burned at an open flame, in which case the hot combustion gases are guided into the housing in order to heat up the reformer and the CO shift converter as well as the various participating gas flows.
U.S. Pat. No. 4,746,329 discloses a methanol reforming reactor of a cylindrical construction consisting of several radially successive annuli. On the underside of the reactor cylinder, a burner unit is situated which may be formed by a catalytic burner. The hot burner exhaust gases are guided through the radially outermost annulus upwards and are then deflected into the radially inner adjacent annulus where they are in a thermal contact with a reforming annulus which adjoins radially on the inside. A top part of the reforming space extends beyond the outer annuli carrying the combustion gas so that a lower operating temperature exists in this area. In this manner, this cooler upper reforming space area is used as a CO shift unit. The reforming space is adjoined on the inside by an evaporator annulus which, in turn, is adjoined radially toward the inside by way of a cylindrical wick by an inner tempering space into which the combustion gases are deflected after flowing downward to the second outermost annulus in the lower cylinder area. The hydrogen-containing anode gas of a fuel cell system is used as fuel for the burner unit. The combustion exhaust gases therefore contain water vapor, of which at least a portion is fed to the evaporator after the combustion exhaust gases flow out at the upper cylinder face.
German Published Patent Application DE 38 03 080 A1 discloses a reforming system for generating synthesis gases containing hydrogen, carbon monoxide and carbon dioxide as well as an operating process therefor. The charged substances are first subjected to an at least one-stage primary reforming, then to a partial oxidation, subsequently to another secondary reforming and finally to a carbon monoxide conversion. In this case, the waste heat of the exothermal carbon monoxide conversion is utilized for the primary vapor reforming, for the purpose of which the corresponding primary reforming stage and the CO conversion stage are in a thermal contact with one another by way of a heat-conducting separating wall.
The present invention is based on the technical problem of providing a system and a process by means of which, during a cold start, the warmed-up normal operation can be reached as fast as possible for the effective implementation of the reforming reaction, for example, for rapidly providing hydrogen for a fuel cell system of a motor vehicle.
By means of the system according to the present invention, an evaporator as well as a main reformer can be heated by way of a respective burner unit, in which a combustible mixture can be burned in a catalytically flameless manner. As a result, the evaporator and the main reformer can be heated directly after a cold start. Simultaneously, at least one prereforming stage is in a thermal contact with a CO shift stage or a CO oxidation stage. Thus, these components, which are coupled in a heat-conducting manner, can be brought jointly to their normal operating temperature by the direct or indirect heating of one of the two components during a heat-up operation at the cold start. In addition, the outlet of the CO removal unit can be connected directly with the inlet of the respective catalytic burner unit. As a result, catalytically combustible gas which exists at the outlet of the CO removal unit can be fed, as required, into the catalytic burner unit for the purpose of being burned there, in order to contribute, alone or in addition to fuel directly fed there, to a fast heating-up of the evaporator and of the main reformer.
An advantageous heating-up operating during a cold start is contained in the process according to the present invention. In a first operating phase, hydrogen or the hydrocarbon to he reformed in normal operation is fed as fuel into the catalytic burner units and is catalytically burned there with an additional feeding of an oxygen-containing gas in order to heat up the burner units to preferably 150.degree. to 350.degree. C., whereby the evaporator and the main reformer are heated correspondingly. In a subsequent second operating phase, the hydrocarbon/water vapor mixture required for the reforming is then started in the preheated evaporator. At this point in time, a portion of hydrocarbon lower in comparison to the later normal operation is fed into the evaporator. The mixture heated in the evaporator is guided through the prereforming unit, whereby the preforming unit and also the CO shift stage and/or the CO-oxidation stage which are in a thermal contact with it are heated. In the preheated main reformer, the still comparatively low amount of fed hydrocarbon can be reformed. The resulting hydrogen-rich reformate heated by the main reformer reaches the CO removal unit which, as a result, is additionally heated up, and from there, arrives in the catalytic burner units, where it may be used as fuel so that the direct feeding of a corresponding fuel can be stopped or at least reduced. During this heating-up operation, the evaporator and/or the main reformer can be operated at a temperature above the normal operating temperature in order to further accelerate the heating-up operation. As soon as the system components have essentially reached their warmed-up operating condition, a change takes place to the normal operation in that the portion of the hydrocarbon to be reformed which is fed into the evaporator is increased correspondingly. The reformate gas, which exists at the outlet side of the CO removal unit will then essentially consist of hydrogen, while carbon monoxide is not present to an interfering degree. Thus, product gas does not have to be fed any longer into the catalytic burner units but can be provided to the intended use, for example, for feeding the anode side of a fuel cell system.
In the case of another embodiment of the present invention, the prereforming unit contains two prereforming stages, and the CO removal unit has a CO shift stage as well as a CO oxidation stage that are serially situated behind one another. The CO shift stage and CO oxidation stage are each in thermal contact with one prereforming stage respectively in order to promote the fast heating-up of these components during the cold start. In addition, devices are provided for shutting off the connection between the CO shift stage and the CO oxidation stage, as well as an intermediate feeding line which leads downstream of these shut-off devices to the CO oxidation stage. By way of the intermediate feeding line, a substance, such as a combustible mixture, can be fed in a targeted manner into the CO oxidation stage. For the operation of this system, a process according the present invention is used in which the connection between the CO shift phase and the CO oxidation phase is separated at the start of the first operating phase and is opened up again at the start of the second operating phase. Simultaneously, during the first operating phase by way of the pertaining intermediate feeding pipe into the CO oxidation stage, a catalytically combustible mixture is introduced there, whereby the CO oxidation stage is directly heated up in an active manner.
In its warmed-up operating condition, another embodiment according to the present invention is used for feeding a fuel cell system with the required hydrogen by supplying the hydrogen-rich reformate gas. During the heating-up operation at a cold start, in order to feed the gas that is present at the outlet of the CO removal unit and which may be damaging to the fuel cell system directly to the catalytic burner units, while by-passing the fuel cell system, a corresponding change-over valve is provided. In this case, the gas present on the outlet side of the CO removal unit is not connected before the end of the second operating phase of the heating-up operation during a cold start with the corresponding inlet of the fuel cell system, while it is otherwise guided into the catalytic burner units.
During another embodiment of the present invention, the system pressure in the reforming gas flow path is raised during the heating-up operation from zero to the normal operating pressure in the normal operation.
In the case of another embodiment of the present invention, the hot combustion exhaust gases of the catalytic burner units are used for heating up a cooling circulation of a fuel cell system whose anode side is fed in the normal operation by hydrogen-rich reformate gas.
A flowing of the hot combustion exhaust gases of the catalytic burner units through additional flow ducts, which are provided in one or several not actively heated system components, contributes to the further acceleration of the heating-up operation.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.